1. Field of the Invention
The present invention relates to a zoom lens suited for a photographic camera for film, a video camera, a digital camera and a still video (SV) camera of the electronic recording type, etc., and more particularly to a zoom lens of the so-called negative lead type in which a negative lens unit leads.
2. Description of Related Art
Heretofore, there has been widely used, as a wide-angle zoom lens, a zoom lens of the so-called negative lead type in which a lens unit of negative refractive power leads, because, in the zoom lens of the negative lead type, the widening of an angle of view is relatively easy.
In the meanwhile, for example, in a zoom lens of the negative lead type having four lens units, at the telephoto end, the first lens unit and the second lens unit constitute a positive group as a whole and the third lens unit and the fourth lens unit constitute a negative group as a whole, so that the entire lens system can be formed into the so-called telephoto type. Therefore, the zoom lens of the negative lead type has such an advantage that it is easy to lengthen the focal length at the telephoto end. This is of course not limited to the four-unit zoom lens.
Among a great number of conventional zoom lenses of the negative lead type, four-unit zoom lenses of the negative lead type have been disclosed in, for example, Japanese Laid-Open Patent Applications No. Sho 60-87312, No. Hei 2-201310, No. Hei 5-241073, No. Hei 4-235515, No. Hei 4-163415, No. Hei 5-313065, No. Sho 58-95315, No. Hei 6-82698, No. Hei 5-19170, No. Hei 7-287168, etc. In these zoom lenses, the variation of magnification is effected by moving at least two lens units of the four lens units.
In the above-mentioned zoom lenses, in order to realize a wide angle of view while retaining a predetermined variable magnification ratio, a negative lens located on the most object side is made of a glass material having a high refractive index. Most of such a glass materials are expensive and are high in specific gravity, causing a considerably large problem in respect of the attainment of low cost and light weight of the zoom lens.
It is an object of the invention to realize the low cost, light weight and small size of a zoom lens while retaining good optical performance thereof.
In the view of the above object, in accordance with an aspect of the invention, there is provided a zoom lens comprising, in order from an object side to an image side, a first lens unit of negative refractive power, a second lens unit of positive refractive power, a third lens unit of negative refractive power, and a fourth lens unit of positive refractive power, wherein the first lens unit has a negative lens located on the most object side thereof, and the negative lens located on the most object side of the first lens unit satisfies the following conditions:
(a) 35 less than xcexd11n less than 65, and
(b) when 35 less than xcexd11n less than 52,
xe2x88x920.013 xcexd11n+2.19 less than N11n less than xe2x88x920.005 xcexd11n+1.92,
when 52 less than xcexd11n less than 60,
1.5 less than N11n less than xe2x88x920.005 xcexd11n+1.92,
when 60 less than xcexd11n less than 65,
1.5 less than N11n less than xe2x88x920.022 xcexd11n+2.94,
where xcexd11n is an Abbe number of a material of the negative lens located on the most object side of the first lens unit, and N11n is a refractive index of the material of the negative lens located on the most object side of the first lens unit.
In the above zoom lens, the first lens unit has a second negative lens other than the negative lens located on the most object side thereof, and the second negative lens satisfies the following conditions:
(c) 35 less than xcexd12n less than 65, and
(d) when 35 less than xcexd12n less than 52,
xe2x88x920.013 xcexd12n+2.19 less than N12n less than xe2x88x920.005 xcexd12n+1.92,
when 52 less than xcexd12n less than 60,
xe2x80x831.5 less than N12n less than xe2x88x920.005 xcexd12n+1.92,
when 60 less than xcexd12n less than 65,
1.5 less than N12n less than xe2x88x920.022 xcexd12n+2.94,
where xcexd12n is an Abbe number of a material of the second negative lens of the first lens unit, and N12n is a refractive index of the material of the second negative lens of the first lens unit.
The above zoom lens becomes, at a telephoto end, a telephoto type in which a plurality of lens units are divided into a lens group on the object side composed of at least one lens unit having a positive refractive power and a lens group on the image side composed of at least one lens unit having a negative refractive power.
In the above zoom lens, during variation of magnification from a wide-angle end to a telephoto end, the first lens unit moves with a locus convex toward the image side, the second lens unit moves toward the object side in such a way as to decrease a separation between the first lens unit and the second lens unit, the third lens unit moves toward the object side in such a way as to increase a separation between the second lens unit and the third lens unit, and the fourth lens unit moves toward the object side in such a way as to decrease a separation between the third lens unit and the fourth lens unit.
The above zoom lens satisfies the following condition:
0.9 less than |f1/fw| less than 1.8
where f1 is a focal length of the first lens unit, and fw is a focal length of the entire zoom lens in a wide-angle end.
In the above zoom lens, the first lens unit comprises, in order from the object side to the image side, a negative lens of meniscus form having a convex surface facing the object side, a negative lens, and a positive lens of meniscus form having a convex surface facing the object side.
In the above zoom lens, the second lens unit and the fourth lens unit move in unison with each other during variation of magnification.
The above zoom lens satisfies the following conditions:
0.65 less than f2/fw less than 1.3
1.2 less than |f3/fw| less than 3.4
where f2 and f3 are focal lengths of the second lens unit and the third lens unit, respectively, and fw is a focal length of the entire zoom lens in a wide-angle end.
The above zoom lens consists of the first to fourth lens units, and satisfies the following condition:
2.1 less than f4/fw less than 8.5
where f4 is a focal length of the fourth lens unit, and fw is a focal length of the entire zoom lens in a wide-angle end.
In the above zoom lens, the second lens unit comprises two positive lenses and one negative lens.
In the above zoom lens, the third lens unit comprises a cemented lens composed of a negative lens and a positive lens.
In the above zoom lens, the fourth lens unit comprises a negative lens and a positive lens, and has at least one aspheric surface.
In the above zoom lens, the second lens unit comprises one positive lens and a positive cemented lens composed of a negative lens and a positive lens.
In the above zoom lens, the fourth lens unit has a plastic aspheric lens.
In the above zoom lens, the second lens unit comprises one positive lens and a positive cemented lens composed of a negative lens and a positive lens, the third lens unit comprises a negative lens and a positive lens, and the fourth lens unit comprises a negative lens and a positive lens, and has a plastic aspheric lens.
In accordance with another aspect of the invention, there is provided a zoom lens comprising, in order from an object side to an image side, a first lens unit of negative refractive power, a second lens unit of positive refractive power, and a third lens unit, wherein all said first to third lens units move during variation of magnification, the first lens unit has a negative lens located on the most object side thereof, and the negative lens located on the most object said of the first lens unit satisfies the following conditions:
(a) 35 less than xcexd11n less than 65, and
(b) when 35 less than xcexd11n less than 52,
xe2x88x920.013 xcexd11n+2.19 less than N11n less than xe2x88x920.005 xcexd11n+1.92,
when 52 less than xcexd11n less than 60,
xe2x80x831.5 less than N11n less than xe2x88x920.005 xcexd11n+1.92,
when 60 less than xcexd11n less than 65,
1.5 less than N11n less than xe2x88x920.022 xcexd11n+2.94,
where xcexd11n is an Abbe number of a material of the negative lens located on the most object side of the first lens unit, and N11n is a refractive index of the material of the negative lens located on the most object side of the first lens unit.
In the above zoom lens, the third lens unit has a negative refractive power.
In accordance with a further aspect of the invention, there is provided a zoom lens comprising, in order from an object side to an image side, a first lens unit of negative refractive power and a second lens unit of positive refractive power, wherein the following conditions are satisfied:
1.1 less than |f1/fw| less than 1.4
0.8 less than |f2/fw| less than 1.1
where f1 and f2 are focal lengths of the first lens unit and the second lens unit, respectively, and fw is a focal length of the entire zoom lens in a wide-angle end.
In the above zoom lens, the second lens unit has a cemented lens composed of a negative lens and a positive lens, and the following condition is satisfied:
20 less than xcexd2pxe2x88x92xcexd2n
where xcexd2p is an Abbe number of a material of the positive lens of the cemented lens of the second lens unit, and xcexd2n is an Abbe number of a material of the negative lens of the cemented lens of the second lens unit.
In accordance with a further aspect of the invention, there is provided a zoom lens comprising, in order from an object side to an image side, a first lens unit of negative refractive power, a second lens unit of positive refractive power, and a third lens unit of negative refractive power, wherein the following conditions are satisfied:
1.1 less than |f1/fw| less than 1.4
0.8 less than |f2/fw| less than 1.1
where f1 and f2 are focal lengths of the first lens unit and the second lens unit, respectively, and fw is a focal length of the entire zoom lens in a wide-angle end.
In the above zoom lens, a stop is disposed adjacent to the third lens unit, the stop moving in unison with the third lens unit.
In the above zoom lens, the stop is disposed on the object side of the third lens unit.
In the above zoom lens, the third lens unit comprises one positive lens and one negative lens, and the following condition is satisfied:
1.5 less than |f3/fw| less than 3.0
where f3 is a focal length of the third lens unit.
In the above zoom lens, the third lens unit has a cemented lens composed of a negative lens and a positive lens, and the following conditions are satisfied:
4.0 less than xcexd3nxe2x88x92xcexd3p less than 12.0
xe2x80x830.05 less than N3pxe2x88x92N3n less than 0.20
where xcexd3n is an Abbe number of a material of the negative lens of the cemented lens of the third lens unit, xcexd3p is an Abbe number of a material of the positive lens of the cemented lens of the third lens unit, N3p is a refractive index of the material of the positive lens of the cemented lens of the third lens unit, and N3n is a refractive index of the material of the negative lens of the cemented lens of the third lens unit.
In accordance with a further aspect of the invention, there is provided a zoom lens comprising, in order from an object side to an image side, a first lens unit of negative refractive power, a second lens unit of positive refractive power, a third lens unit of negative refractive power, and a fourth lens unit of positive refractive power, wherein the following conditions are satisfied:
1.1 less than |f1/fw| less than 1.4
0.8 less than |f2/fw| less than 1.1
where f1 and f2 are focal lengths of the first lens unit and the second lens unit, respectively, and fw is a focal length of the entire zoom lens in a wide-angle end.
The above zoom lens consists of the first to fourth lens units, the fourth lens unit comprises one positive lens and one negative lens, and the following condition is satisfied:
2.5 less than |f4/fw| less than 8.0
where f4 is a focal length of the fourth lens unit.
In the above zoom lens, the fourth lens unit has at least one aspheric surface.
In accordance with a further aspect of the invention, there is provided an image pickup apparatus comprising the above zoom lens, a photosensitive member, and means for supporting the zoom lens and the photosensitive member.
In accordance with a further aspect of the invention, there is provided an image projection apparatus comprising the above zoom lens, a light source, and an image forming element, and arranged to project an image.